Chemical fluid deposition method for the formation of metal and metal alloy films on patterned and unpatterned substrates

ABSTRACT

Methods are described for depositing a film or discontinuous layer of discrete clusters, of material (e.g., metals, metal mixtures or alloys, metal oxides, or semiconductors) on the surface of a substrate, e.g., a patterned silicon wafer, by i) dissolving a precursor of the material into a supercritical or near-supercritical solvent to form a supercritical or near-supercritical solution; ii) exposing the substrate to the solution, under conditions at which the precursor is stable in the solution; and iii) mixing a reaction reagent into the solution under conditions that initiate a chemical reaction involving the precursor, thereby depositing the material onto the solid substrate, while maintaining supercritical or near-supercritical conditions. The invention also includes similar methods for depositing material particles into porous solids, and films of materials on substrates or porous solids having material particles deposited in them. The invention also covers methods of preparing a plated substrate by depositing a catalytic layer followed by a plating layer.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims benefit of priority from U.S. Provisional PatentApplication Serial No. 60/163,163, filed on Nov. 2, 1999, and from U.S.Provisional Patent Application No. 60/223,839, filed on Aug. 8, 2000,both of which are incorporated herein by reference in their entirety.

FIELD OF THE INVENTION

The invention relates to methods for depositing materials onto substratesurfaces or into porous solids.

BACKGROUND OF THE INVENTION

Thin films of materials such as metals, semiconductors, or metal oxideinsulators are of great importance in the microelectronics industry.Fabrication of integrated circuits involves formation of high puritythin films, often with multiple layers, on patterned substrates. One ofthe most common methods for producing thin films is chemical vapordeposition (CVD). In thermal CVD, volatile precursors are vaporizedunder reduced pressure at temperatures below their thermal decompositiontemperature and transported by means of a carrier gas into an evacuatedchamber containing a substrate. The substrate is heated to hightemperatures, and thermolysis at or adjacent to the heated substrateresults in the surface deposition of the desired film. For a generalreference on CVD see: Hitchman et al., eds., Chemical Vapor DepositionPrinciples and Applications (Academic Press, London, 1993).

Thin films have also been formed using supercritical fluids. Forexample, Murthy et al. (U.S. Pat. No. 4,737,384) describes a physicaldeposition method in which a metal or polymer is dissolved in a solventunder supercritical conditions and as the system is brought tosub-critical conditions the metal or polymer precipitates onto anexposed substrate as a thin film. Sievers et al. (U.S. Pat. No.4,970,093) describes a standard CVD method in which organometallic CVDprecursors are delivered to a conventional CVD reactor by dissolving theprecursors in a supercritical fluid solvent. The solvent is expanded toproduce a fine precursor aerosol which is injected into the CVD reactorunder standard CVD conditions, i.e., pressures less than or equal to 1atmosphere, to deposit a thin film on a substrate.

Louchev et al. (J. Crystal Growth, 155:276-285, 1995) describes thetransport of a precursor to a heated substrate (700 K) in asupercritical fluid where it undergoes thermolysis to yield a thin metal(copper) film. Though the process takes place under high pressure, thetemperature in the vicinity of the substrate is high enough that thedensity of the supercritical fluid approaches the density of aconventional gas. The film produced by this method had an atomic copperconcentration of approximately 80% (i.e., 20% impurities). Bouquet etal. (Surf and Coat. Tech., 70:73-78, 1994) describes a method in which ametal oxide is deposited from a supercritical mixture of liquid and gasco-solvents at a temperature of at least 240° C. The thin film forms asa result of thermolysis at a substrate heated to at least 290° C.

The formation of alloys from multiple pure metal components and filmscontaining multiple pure metal components is also of interest inmicroelectronic applications and device fabrication for the formation offilms exhibiting, e.g., gigantic magneto resistance (GMR), increasedresistance to electromigration and for modification of electricalconductivity, and for the formation of other functional layers inintegrated circuits. Alloying is also used to tailor rate andselectivity for reactions over supported catalysts, improve theresistance of metal membranes to hydrogen embrittlement and to increasethe hardness and corrosion resistance of barrier coatings. Mixed metalfilms are typically produced by physical deposition methods such as ionsputtering, which is a line-of-sight technique. In principle, CVD canalso be used to produce alloy films using a combination of metalprecursors. Such deposition, however, would be limited by the relativevolatilities of the precursors making precise control of multi-componentfeed streams across the composition range difficult to achieve.Moreover, attainment of a desired composition would also depend on therelative rates of decomposition.

Thin films of palladium (Pd) and its alloys are used in technologicallyimportant applications such as catalysis, gas sensors, and H₂permselective membranes for use in gas separation and in integratedreaction/separation schemes. Moreover, Pd is a common noble metal inmicroelectronics, where it is used as a contact material in integratedcircuits and as a seed layer for the electroless deposition of otherinterconnect metals. Pd films can be prepared by vacuum sputtering andelectroplating. However, such techniques are generally limited to planarsurfaces, limiting their applicability to applications inmicroelectronics where shrinking device dimensions require efficientfilling of deep sub-micron, high-aspect ratio features.

High purity Pd thin films can be deposited by CVD using organopalladiumcompounds containing various classes of ancillary ligands as precursors.However, to maintain acceptable purity and deposition rates,temperatures usually exceed 200° C. Moreover, because CVD is oftenmass-transport limited, the deposited films are expected to benon-uniform, thereby limiting efficient pore-filling and/or conformalcoverage of complex surfaces. Consequently, palladium CVD has not yetbeen commercialized.

Copper is also used in technologically important applications, includinginterconnect structures in microelectronic devices. Current methods ofdepositing copper, such as CVD and sputtering, have not been shown toprovide uniform filling of very narrow (˜150 nm and less), high aspectratio trenches or vias. As a result, copper CVD has not been practicedcommercially for these applications. Other applications for copperinclude printed wiring boards.

SUMMARY OF THE INVENTION

The invention features new methods for depositing a material, e.g., athin film of a pure metal, a mixed metal, or a metal alloy, or a layer,e.g., a discontinuous layer of discrete uniformly distributed clusters,onto a substrate surface or into a porous solid substrate. The methodsare generally referred to herein as chemical fluid deposition (CFD). CFDinvolves dissolving a precursor of the material to be deposited into asolvent under supercritical or near-supcrcritical conditions andexposing the substrate (or porous solid) to the solution. A reactionreagent is then mixed into the solution and the reaction reagentinitiates a chemical reaction involving the precursor, therebydepositing the material onto the substrate surface (or within the poroussolid). Use of a supercritical solvent in conjunction with a reactionreagent produces high purity thin films, e.g., metal or metal alloyfilms, or layers of discrete high purity metal or metal alloy clusters,at temperatures that can be lower than conventional CVD temperatures.The substrate surface can include one or more layers, which may bepatterned. When patterned substrates are used, e.g., having deepsub-micron, high-aspect ratio features such as trenches, CFD can provideuniform conformal coverage and uniform filling of the features.

The invention also features a two-step process that involves (1) thedeposition of a catalytic seed layer, e.g., of palladium, platinum, orcopper, by CFD, followed by (2) plating, e.g., electroless orelectrolytic plating, or additional CFD, of more of the same metal oranother metal or alloy. The seed layer need not be continuous, i.e., theseed layer can be made of clusters of deposited material, but theisolated catalytic seed clusters should be distributed uniformly in anypatterns, e.g., trenches or invaginations, in the surface of thesubstrate. The surface can be functionalized prior to deposition usingcoupling agents, e.g., chlorotrimethoxysilane, or example, to controlthe concentration and location of the seed layer deposit.

In another two-step process, a seed layer and thin film is createdsimultaneously by a first thermal disproportionation step using aprecursor such as copper (e.g., Cu(I)), followed by the addition of areaction reagent such as H₂ to reduce the products of thedisproportionation reaction in a CFD method to obtain high yielddeposition of the precursor onto a substrate.

In general, in one aspect, the invention features a method fordepositing a film of a material, e.g., a metal, mixture of metals, metalalloy, metal oxide, metal sulfide, insulator; or semiconductor, onto thesurface of a substrate, e.g., a silicon wafer, by i) dissolving aprecursor of the material into a solvent, e.g., carbon dioxide, undersupercritical or near-supercritical conditions to form a supercriticalor near-supercritical solution; ii) exposing the substrate to thesolution under conditions at which the precursor is stable in thesolution; and iii) mixing a reaction reagent, e.g., hydrogen, intosolution under conditions that initiate a chemical reaction involvingthe precursor, e.g., a reduction, oxidation, or hydrolysis reaction,thereby depositing the material onto the surface of the substrate, whilemaintaining supercritical or near-supercritical conditions.

For example, the method can be conducted so that the temperature of thesubstrate is maintained at no more than 200, 225, 250, 275, or 300° C.,the solvent has a reduced temperature between 0.8 and 2.0, e.g., 1.0,1.2, 1.4, 1.6, or 1.8, the solvent has a density of at least 0.1 g/cm³,e.g., 0.125, 0.15, 0.175, or 0.2 g/cm³, the solvent has a density of atleast one third of its critical density, or so that the solvent has acritical temperature of less than 150° C. In addition, the method can becarried out so that the temperature of the substrate measured in Kelvinis less than twice the critical temperature of the solvent measured inKelvin, or so that the temperature of the substrate measured in Kelvindivided by the average temperature of the supercritical solutionmeasured in Kelvin is between 0.8 and 1.7. The method can also beconducted such that the average temperature of the supercriticalsolution is different from the temperature of the substrate.

In some embodiments, the material comprises multiple metals and theprecursor comprises multiple precursors for the multiple metals.Furthermore, the material can be a homogeneous or inhomogeneous mixtureof multiple metals, for example, the material can be a platinum/nickelmixture or alloy, or a copper mixture or alloy. Moreover, gradients ofvarying concentrations of individual metals may be created throughout athin film.

The substrate can be a patterned substrate, such as one used in themicroelectronics industry. The patterned substrate can have submicronfeatures, which may have an aspect ratio greater than about 2, greaterthan about 3, or greater than about 10. The material can be deposited toconformally cover the features. In one embodiment, the substrate is apatterned silicon wafer and the material is palladium or a palladiumalloy that conformally covers the patterned features. In anotherembodiment, the substrate is a patterned silicon wafer and the materialis copper or a copper alloy that conformally covers or fills thepatterned features.

In another aspect, the invention features an integrated circuitincluding a patterned substrate having submicron features and a filmincluding palladium or copper conformally covering the features. Theaspect ratio of the patterned features can be greater than about 2,greater than about 3, or greater than about 10.

The invention also features a method for depositing material within amicroporous or nanoporous solid substrate by dissolving a precursor ofthe material into a solvent under supercritical or near-supercriticalconditions to form a supercritical or near-supercritical solution; ii)exposing the solid substrate to the solution under conditions at whichthe precursor is stable in the solution; and iii) mixing a reactionreagent into the solution under conditions that initiate a chemicalreaction involving the precursor, thereby depositing the material withinthe solid substrate, while maintaining supercritical ornear-supercritical conditions. For example, this method can be conductedsuch that the temperature of the solid substrate is maintained at nomore than 300, 275, 250, 225, 210, 200, or 190° C.

In another aspect, the invention features a film of a material, e.g., ametal, metal mixture, metal alloy, or semiconductor, on a substrate, thecoated substrate itself, and microporous or nanoporous solid substrateshaving such materials deposited on and within them. One embodiment is ametal or metal alloy membrane formed within a porous solid substrate.These new substrates may or may not be prepared by the new methods. In afurther aspect, the invention features an integrated circuit includingthe new substrates, which may be prepared by the new method.

In other embodiments, the invention features a method of depositing aseed layer of a material onto a substrate by i) dissolving a precursorof the material into a solvent to form a supercritical ornear-supercritical solution; ii) exposing the substrate to the solutionunder conditions at which the precursor is stable in the solution; andiii) mixing a reaction reagent into the solution under conditions thatinitiate a chemical reaction involving the precursor, wherein thematerial is deposited as a seed layer onto the surface of the substratewhen the substrate and the reaction reagent are in contact with thesolution, while maintaining supercritical or near-supercriticalconditions. The method can further include a step of depositing a metalfilm on the seed layer, e.g., by CFD.

The invention also includes a method of depositing a material onto asubstrate by i) depositing a seed layer onto the substrate; ii)dissolving a precursor of the material into a solvent to form asupercritical or near-supercritical solution; iii) exposing thesubstrate and seed layer to the solution under conditions at which theprecursor is stable in the solution; and iv) mixing a reaction reagentinto the solution under conditions that initiate a chemical reactioninvolving the precursor, wherein the material is deposited onto the seedlayer on the surface of the substrate when the substrate and thereaction reagent are in contact with the solution, while maintainingsupercritical or near-supercritical conditions.

A variation includes a method of depositing a material onto a substrateby i) dissolving a precursor of the material into a solvent to form asupercritical or near-supercritical solution; ii) depositing a seedlayer from the precursor by reduction of the precursor; and iii) mixinga reaction reagent into the solution under conditions that initiate achemical reaction involving the precursor or reduction or decompositionproducts of the precursor, wherein the material is deposited onto theseed layer on the surface of the substrate when the substrate and thereaction reagent are in contact with the solution, while maintainingsupercritical or near-supercritical conditions.

In another embodiment, the invention features a method of depositing amaterial onto a substrate by i) dissolving a precursor of the materialinto a solvent to form a supercritical or near-supercritical solution;and ii) depositing the material by simultaneous thermal reduction (e.g.,disproportionation or thermolysis) and reaction with a reaction reagentin the solution under conditions that initiate a chemical reactioninvolving the precursor or reduction or decomposition products of theprecursor, wherein the material is deposited on the surface of thesubstrate when the substrate and the reaction reagent are in contactwith the solution, while maintaining supercritical or near-supercriticalconditions.

The invention also includes a method of depositing a material onto asubstrate by i) dissolving a precursor or mixture of precursors of thematerial into a solvent to form a supercritical or near-supercriticalsolution; and ii) adding a reaction reagent in the solution underconditions that initiate a chemical reaction involving the precursor orreduction or decomposition products of the precursor, wherein thematerial is deposited on the surface of the substrate when the substrateand the reaction reagent are in contact with the solution, whilemaintaining supercritical or near-supercritical conditions.

As used herein, a “supercritical solution” (or solvent) is one in whichthe temperature and pressure of the solution (or solvent) are greaterthan the respective critical temperature and pressure of the solution(or solvent). A supercritical condition for a particular solution (orsolvent) refers to a condition in which the temperature and pressure areboth respectively greater than the critical temperature and criticalpressure of the particular solution (or solvent).

A “near-supercritical solution” (or solvent) is one in which the reducedtemperature (actual temperature measured in Kelvin divided by thecritical temperature of the solution (or solvent) measured in Kelvin)and reduced pressure (actual pressure divided by critical pressure ofthe solution (or solvent)) of the solution (or solvent) are both greaterthan 0.8 but the solution (or solvent) is not a supercritical solution.A near-supercritical condition for a particular solution (or solvent)refers to a condition in which the reduced temperature and reducedpressure are both respectively greater than 0.8 but the condition is notsupercritical. Under ambient conditions, the solvent can be a gas orliquid. The term solvent is also meant to include a mixture of two ormore different individual solvents.

The “aspect ratio” of a feature on a patterned substrate is the ratio ofthe depth of the feature and the width of the feature.

Unless otherwise defined, all technical and scientific terms used hereinhave the same meaning as commonly understood by one of ordinary skill inthe art to which this invention belongs. Although methods and materialssimilar or equivalent to those described herein can be used in thepractice or testing of the present invention, the preferred methods andmaterials are described below. All publications, patent applications,patents, and other references mentioned herein are incorporated byreference in their entirety. In case of conflict, the presentspecification, including definitions, will control. In addition, thematerials, methods, and examples are illustrative only and not intendedto be limiting.

The invention includes a number of advantages, including the use ofprocess temperatures that are much lower than conventional CVDtemperatures. A reduction in process temperature is advantageous inseveral respects: it aids in the control of depositions, minimizesresidual stress generated by thermal cycling in multi-step devicefabrication that can lead to thermal-mechanical failure, minimizesdiffusion and reaction of the incipient film with the substrate and/oradjacent layers, renders the deposition process compatible withthermally labile substrates such as polymers, and suppressesthermally-activated side-reactions such as thermal fragmentation ofprecursor ligands that can lead to film contamination. Thus, the filmsproduced by the process are substantially free of impurities, e.g.,ligand-derived impurities.

An additional advantage of the invention is that it obviates the CVDrequirement of precursor volatility since the process is performed insolution. Thus, the process can be conducted at fluid phase precursorconcentrations between 10 and 10,000 times or more higher thanconventional gas phase processes, which mitigates mass transferlimitations and promotes conformal coverage. Furthermore, since theprocess is performed under supercritical or near-supercriticalconditions, the diffusivity of precursors dissolved in solution isincreased relative to liquid solutions, thereby enhancing transport ofprecursor and reaction reagent to, and decomposition products away from,the incipient film. The supercritical fluid is also a good solventfor-ligand-derived decomposition products, and thus facilitates removalof potential film impurities and increases the rate at which materialforms on the substrate in cases where this rate is limited by thedesorption of precursor decomposition products. In addition, since thereactants are dissolved into solution, precise control of stoichiometryis possible.

This latter consideration permits mixed metal depositions, e.g., alloys,because transport is based on solubility rather than volatility. Thus,multicomponent films can be prepared by co-reduction of appropriateorganometallic compounds, and the composition of the multicomponent filmcan be controlled directly by stoichiometric adjustments to the fluidphase precursor concentrations.

Another advantage of the invention is that the supercritical solution isusually miscible with gas phase reaction reagents such as hydrogen. As aresult, gas/liquid mass transfer limitations common to reactions inliquid solvents are eliminated, and so excess quantities of the reactionreagent can easily be used in the reaction forming the material. Thus,the techniques produce high quality metal and metal alloy deposits ofprecisely tailored composition in the form of thin films, conformalcoatings on topologically complex surfaces, uniform deposits within highaspect ratio features, and both continuous and discreet deposits withinmicroporous supports. Moreover, the absence of surface tension inherentto supercritical solutions ensures complete wetting of tortuoussurfaces. In addition, the elimination of the volatility requirementremoves constraints on precursors, and enables the use ofnon-fluorinated precursors that contain environmentally benign ligands.

Other features and advantages of the invention will be apparent from thefollowing detailed description, and from the claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic of a double flange, cold-wall reactor for use incertain of the methods described herein.

FIG. 2A is an X-ray photoelectron spectroscopy (XPS) survey spectra (75°take-off angle) of a platinum film deposited on a silicon wafer usingCFD. Results are shown after sputter cleaning with Ar⁺ ions to removeatmospheric contamination.

FIG. 2B is an XPS survey spectra (75° take-off angle) of a palladiumfilm deposited on a silicon wafer using CFD. Results are shown aftersputter cleaning with Ar⁺ ions to remove atmospheric contamination. Theinset is an expansion of the C_(1s) region of the spectra.

FIGS. 3A and 3B are SEM micrographs of copper films grown on palladiumseeded, patterned silicon wafers using bis(2,2,6,6-tetramethyl3,5-heptanedionato) copper(II) (Cu(TMHD)₂) in a hot-wall reactor. Allfeatures are approximately 1 μm deep.

FIG. 3C is X-ray diffraction data for thin films of Ni/Pt alloys. Theinset is a plot of the alloy composition determined from the x-ray dataversus the feed composition for the metal precursors.

FIGS. 4A-4D are SEM micrographs of palladium films grown on patterned Siwafers using a 0.26 wt. % solution of CpPd(π³-C₄H₇) in CO₂ at 138 barand 60° C. All features are approximately 1 μm deep with the widths ofthe etched features as follows: (4A and B) 0.12 μm, (4C) 0.7 μm and (4D)0.3 μm (measured at ½ the trench height).

FIGS. 5A-5D are additional SEM micrographs of conformal trench fillingby CFD. In particular, the FSEM micrographs show nickel films depositedonto Pd seeded, etched Si wafers by hydrogen reduction ofbis(cyclopentadienyl)nickel (NiCp₂) in CO₂ in either (5A) a batchreactor at 140 bar and 60° C., or (5B-5D) a continuous-flow reactor at60° C. and 180 bar. Ni film conformality and cohesion is evident in thethick film that has delaminated from the substrate during wafer fractureprior to analysis.

FIG. 6A is a SEM micrograph of a seed layer of palladium clusters on asilicon wafer.

FIG. 6B is a SEM micrograph of a close-up view of the micrograph in FIG.6A, showing several palladium clusters within a trench only 140 nm wide.

FIGS. 7A and 7B are SEM micrographs showing copper deposition withintrenches on a substrate that are 165 nm wide by 1100 nm deep (7A) withina trench that is 235 nm wide by 600 nm deep (7B) by a two step processconsisting of deposition of a Pd seed layer using CFD followed byelectroless deposition.

FIG. 8A is a SEM micrograph of a copper film deposited via reduction ofcopper (II) bishexafluoroacetylacetonate (Cu(hfac)₂) onto a palladiumseeded silicon wafer.

FIG. 8B is an XPS survey spectrum of the copper film of FIG. 8A. Resultsare shown after sputter cleaning with Ar⁺ ions to remove atmosphericcontamination.

FIG. 9A is a SEM micrograph of a copper film deposited via reduction ofcopper (I) (hexafluouroacetylacetonate)(2-butyne) (Cu(hfac)(2-butyne))onto a palladium seeded silicon wafer.

FIG. 9B is an XPS survey spectrum of the copper film of FIG. 9A. Resultsare shown after sputter cleaning with Ar⁺ ions to remove atmosphericcontamination.

FIGS. 10A to 10D are SEM micrographs of Ni films deposited onto etchedSi wafers using a batch reactor at 140 bar and 60° C. (10A and B) or acontinuous-flow reactor at 60° C. and 180 bar (10C and D).

DETAILED DESCRIPTION

Chemical Fluid Deposition (CFD) is a process by which materials (e.g.,metals, metal mixtures or alloys, metal oxides, or semiconductors) aredeposited from a supercritical or near-supercritical solution viachemical reaction of soluble precursors. Desired materials can bedeposited on a substrate as a high-purity (e.g., better than 95, 97, oreven 99%) or multicomponent metal mixture or alloy thin film (e.g., lessthan 10, 8, 5, or 3 microns), a conformal coating on a topologicallycomplex surface, and as both a continuous and/or discreet deposit withina microporous support. The substrate can be, e.g., a metal, asemiconductor, or a polymer, can be patterned with a complex surface,and can include one or more previously formed layers or coatings. Thesupercritical fluid transports the precursor to the substrate surfacewhere the reaction takes place and transports ligand-deriveddecomposition products away from the substrate thereby removingpotential film impurities. Typically, the precursor is unreactive byitself and a reaction reagent (e.g., a reducing or oxidizing agent) ismixed into the supercritical solution to initiate the reaction, whichforms the desired materials. The entire process takes place in solutionunder supercritical conditions. The process provides high-purity filmsat various process temperatures under about 300° C. (e.g., below 275,250, 225, 210, 200, 175, 150, 125, 100, 80, 60, or even 40° C.),depending on the precursors, solvents, and process pressure used.

CFD can be used, for example, to deposit platinum (Pt) and palladium(Pd) films onto silicon wafers or fluoropolymer substrates. In theseexamples, process temperatures of as low as 80° C. provide a film puritythat can be better than 99%. CFD can also be used for deposition ofmulticomponent alloy films, e.g., nickel/platinum (Ni/Pt) alloys ofincreasing Ni composition spanning the composition range between the twoelements. The composition of the alloy is dictated by the stoichiometricratio of the precursors in supercritical CO₂ solution. Furthermore, CFDcan be used to provide complete conformal and uniform coverage ofpatterned substrates such as patterned silicon (Si) wafers havingfeature sizes as small as, e.g., 0.1 microns wide by 1.0 micron deep. Acomplete description of such examples and others are given below.

CFD can also be used to deposit materials into mesoporous or microporousinorganic solids. Examples include the metallization of nanometer-sizepores in catalyst supports such as silicalites and amorphous mesoporousaluminosilicate molecular sieves. Supercritical fluids have gas-liketransport properties (e.g., low viscosity and absence of surfacetension) that ensure rapid penetration of the pores. Uniform depositionthroughout the pores is further facilitated by independent control ofthe transport (via solution) and deposition (via reaction reagent)mechanisms in CFD. In addition, CFD can be used to prepare metal ormetal alloy membranes formed within porous substrates such as alumina.By contrast, metallization of porous substrates by CVD often results inchoking of the pores by rapid deposition at the pore mouth.

General Method of Using a Hot-wall Reactor

A batch CFD run in a “hot-wall” reactor involves the following generalprocedure. A single substrate and a known mass of precursor (which caninclude precursor materials for multiple components) are placed in areaction vessel (e.g., a stainless steel pipe), which is sealed, purgedwith solvent, weighed, and immersed in a circulating controlledtemperature bath (thereby heating the walls of the reactor). The vesselis then filled with solvent using a high-pressure transfer manifold. Thecontents of the reactor are mixed using a vortex mixer and conditionsare brought to a specified temperature and pressure at which the solventis a supercritical or near-supercritical solvent. The mass of solventtransferred into the reaction vessel is determined gravimetrically usingstandard techniques. The vessel is maintained at this condition (atwhich the precursor is unreactive) for a period of time, e.g., up to onehour or longer, sufficient to ensure that the precursor has completelydissolved and that the reaction vessel is in thermal equilibrium.

A reaction reagent is then transferred through a manifold connected tothe reaction vessel. The reaction reagent can be a gas or a liquid, or agas,. liquid, or solid dissolved in a supercritical solvent. Thetransfer manifold is maintained at a pressure in excess of that of thereaction vessel. The mass of reaction reagent transferred into thereaction vessel is usually in molar excess relative to the precursor.The reaction is typically carried out for at least one hour, althoughthe reaction may be completed in much less than one hour, e.g., lessthan 30, 20, 10, or 5 minutes, or less than 180, 120, 60 or 30 seconds.The optimal length of reaction time can be determined empirically. Whenthe reactor has cooled, the substrate is removed and can be analyzed.

A variation of the batch process involves heating the substrate byplacing it on a resistance heater within the high-pressure vessel. Theseexperiments are conducted within a dual flange “cold-wall” reactor asdescribed in further detail below.

A continuous CFD process is similar to the above batch method exceptthat known concentrations of the supercritical (or near-supercritical)solution and reaction reagent are taken from separate reservoirs andcontinuously added to a reaction vessel containing multiple substratesas supercritical solution containing precursor decomposition products orunused reactants is continuously removed from the reaction vessel. Theflow rates into and out of the reaction vessel are made equal so thatthe pressure within the reaction vessel remains substantially constant.The overall flow rate is optimized according to the particular reaction.Prior to introducing precursor-containing solution into the reactionvessel, the reaction vessel is filled with neat solvent (which is thesame as the solvent in the precursor solution) at supercritical ornear-supercritical pressures and is heated to supercritical ornear-supercritical temperatures. As a result, supercritical ornear-supercritical conditions are maintained as the precursor-containingsolution is initially added to the reaction vessel.

Solubility of the precursor at the reaction conditions can be verifiedin a variable volume view cell, which is well known in the art (e.g.,McHugh et al, Supercritical Fluid Extraction: Principles and Practice;Butterworths, Boston, 1986). Known quantities of precursor andsupercritical solvent are loaded into the view cell, where they areheated and compressed to conditions at which a single phase is observedoptically. Pressure is then reduced isothermally in small incrementsuntil phase separation (either liquid-vapor or solid-vapor) is induced.

The temperature and pressure of the process depend on the reactants andchoice of solvent. Generally, temperature is less than 300° C. (e.g.,275, 250, 225, 210, 200, 180, 160, 150, or 125° C.) and often less than100° C., while the pressure is typically between 50 and 500 bar (e.g.,between 100 and 400, 100 to 150, or 150 to 250 bar). A temperaturegradient between the substrate and solution can also be used to enhancechemical selectivity.

General Two-step Method

A two-step method can be used in situations where a desired metal ormetal alloy, such as copper, is not readily deposited on a substrate,especially a patterned substrate, using conventional plating techniquesor even the new CFD methods described herein. In these situations, auniform seed layer, e.g., of clusters, is prepared from a material, suchas Pd, Pt, or Cu, which can activate the substrate and serve as acatalytic site on which the desired metal or metal alloy can bedeposited in the second step.

The first step of the method can be similar to the general methoddescribed above with respect to the hot-wall reactor. The first step canalso be achieved by a variety of other methods described in furtherdetail below with respect to use in a “cold-wall” reactor.

As shown in the examples described herein, reactive depositions ofprecursors from supercritical fluids can yield a seed layer of small Pdclusters that is an active catalyst for the second step, e.g., theelectroless deposition of another metal, such as Cu, within sub 150 nmwide, high-aspect ratio features. The seed layer created by the firststep need not be continuous, but the isolated catalytic seed clustersmust be distributed uniformly in the features of a patterned substrate,i.e., the seed layer must uniformly cover the walls of any trenches. Theclusters, rather than a thin film, are obtained by controlling theconcentration of the metal precursor in the supercritical fluid orsolvent. For example, discrete clusters are obtained using precursorconcentrations of less than 0.1 percent by mass, whereas thin films areformed using precursor concentrations of greater than 0.1 percent, andtypically 0.2, 0.3, 0.5 percent, or greater Seed layers comprisingclusters rather than a continuous film of material are often preferred,because it is the second layer, e.g., a film of copper, that has thedesired function(s), e.g., conductivity, and the seed layer caninterfere with the desired function. Therefore, it is often beneficialto minimize the amount of precursor material used to deposit the seedlayer, and uniformly distributed clusters have less mass than acontinuous film covering the same surface area.

Variations of the process include control of Pd cluster size withspecific chemical functional groups introduced onto the surface of thesubstrate via coupling agents, such as silane coupling agents. A widevariety of coupling agents is available commercially, e.g., from Gelest,Inc., Tullytown, Pa. In addition, functional groups can be bound tosubstrate surfaces using a variety of other approaches. For example,functional groups can be bound to surfaces using plasma surfacemodification, e.g., as described in U.S. Pat. Nos. 4,900,618 and4,910,072 to Monsanto, or by using polymers with specific functionalgroups, e.g., as described in U.S. Pat. No. 4,869,930 issued to IBM. Therole of the functional group is to bind to the Pd precursor during itsintroduction, and the coupling agents, and thus the functional groups,can be selectively bound to the surface in discrete locations, e.g., bymasking or selective removal techniques. The bound organometallic, e.g.,bound in only certain regions of the substrate, is subsequently reducedin the supercritical solvent or after removal of the supercriticalsolvent. By allowing the precursor to bind to the functional groupsprior to reduction, one can carefully control the size of the clustersas well as the location of the bound precursor, which is then reducedonly after any unbound precursor is removed from the substrate surface.

In addition, Cu seed layers, e.g., in the form of clusters, can bedeposited by the reduction of various organocopper compounds insupercritical fluids. Cu precursors, include, but are not limited to,Cu(II) beta diketonates such as Cu(hfac)₂, Cu(hfac)₂ hydrate,bis(6,6,7,7,8,8,8-heptafluoro-2,2-dimethyl-3-5-octanedionate)copper(II),bis(2,2,6,6,-tetramethyl-3,5-heptanedionato)copper(II), and Copper (II)acetylacetonate. Cu(I) precursors, include, but are not limited to,(β-diketonate)CuL compounds where L is a ligand that can includealkynes, phosphines, olefins, cyclooctadiene, and vinyl trimethysilane.The Cu seed layer can be formed from the precursor by either mixing witha reaction reagent, by thermal disproportionation, or by other reductionpathways.

The second step of the two-step method can be a standard electroless orelectrolytic plating step, or subsequent CFD as described herein. Oneembodiment is the use of commercial plating baths such as the M-Copper85 plating system from MacDermid Incorporated or other plating systemsfrom other sources. Such plating methods are described, e.g., in Dubin,et al., J. Electrochem. Soc., 144:898-098 (1997).

General Method of Using a Cold-wall Reactor

A variation of the batch process described above involves a dual flange,“cold-wall” reactor in which the substrate is placed on top of aresistively heated stage. As shown in FIG. 1, the reactor housing ismade of a stainless steel top flange 11 a, and a stainless steel bottomflange 11 b connected to top flange, e.g., via bolts (not shown). Thetop and bottom flanges are sealed with an O-ring seal 20, e.g., a Bunarubber O-ring. The inside of the top flange 11 a and the surface of thebottom flange 11 b are both lined with a TEFLON liner 12 to create aninternal chamber 11 c. A heated substrate stage 15, which can be heated,e.g., by a nickel-chromium resistance heater 16 potted into the stagewith potting cement, is arranged within this chamber 11 c.

The substrate 14 to be coated is placed on stage 15. A thermocouple 13located on the stage 15, and preferably contacting the substrate 14, isconnected to a temperature controller through high pressure feed-through(wires not shown), and is used to monitor and control the temperature ofthe substrate. Reactor housing 10 also includes a high pressure (e.g.,{fraction (1/16)}″) line 17 for reactant feed, a first port 18 forrupture disk, feeds, outlets, thermocouples, pressure measurement, etc.,and a second port 19 for rupture disk, feeds, outlets, thermocouples,pressure measurement, etc. In addition, the housing has a third port 21for high pressure feed-through (wires not shown).

The temperature of the stage 15, and the substrate contacting the stage,is controlled by regulating power delivered to the heater using atemperature controller (e.g., PID controller). In a typical experiment,a single substrate 14 is placed on stage 15, and a known mass ofprecursor (which can include precursor materials for multiplecomponents) is placed in the reactor 10. The reactor is then heated tothe desired temperature, typically 40-80° C. and filled with solventusing a high-pressure manifold or a computer controlled syringe pump,and the contents are brought to a specified temperature and pressure atwhich the solvent is a supercritical or near-supercritical solvent. Thevessel is maintained at this condition (at which the precursor isunreactive) for a period of time, e.g., up to one hour or longer,sufficient to ensure that the precursor has completely dissolved. Thesubstrate is then heated to a specific temperature, typically 150-250°C. on the stage, which is higher than the bulk temperature of thesupercritical solvent/precursor mixture.

In one embodiment, a reaction reagent is then transferred through amanifold connected to the reaction vessel. The reaction reagent can be agas or a liquid, or a gas, liquid, or solid dissolved in a supercriticalsolvent. The transfer manifold is maintained at a pressure in excess ofthat of the reaction vessel. The mass of reaction reagent transferredinto the reaction vessel is usually in molar excess relative to theprecursor. The reaction is typically carried out for at least one hour,although the reaction may be completed in much less than one hour, e.g.,less than 30, 20, 10, or 5 minutes, or less than 180, 120, 60, or 30seconds. The optimal length of reaction time can be determinedempirically. When the reactor has cooled, the substrate is removed andcan be analyzed. This method can be employed as a single-step depositionon an unseeded substrate or as a two-step method as described above,where a catalytic seed layer is first deposited on the substrate and ametal film of the same or different composition is deposited on theseeded substrate.

Variations of this method, which apply in both the cold-wall andhot-wall reactors, include (i) deposition of the seed layer by CFDfollowed by metal deposition by other techniques including CVD orelectroless or electrolytic plating; (ii) deposition of a seed layerusing any technique including sputtering, CVD, electroless plating,thermolysis, or other reactions at the substrate surface followed byCFD; and (iii) deposition of both the seed layer and metal film by CFD;and sequential and/or simultaneous combinations of methods.

One embodiment of deposition in a cold-wall or hot-wall reactor includesmultiple reactions from one precursor or a precursor and its reactionproducts. For example, Cu films can be deposited onto a variety ofsubstrates, e.g., silicon, metals, glasses, polyimides, various oxidessuch as silicon oxides, and nitrides such as titanium nitrides, fromCu(I) precursors of the general type (β-diketonate)CuL_(n), where L is aLewis base, and n is 1 or 2. A precursor, or mixture of precursors ofthis type, are dissolved in CO₂ as described above. The temperature ofthe substrate is then increased to initiate a thermal disproportionationreaction that yields a copper deposit according to the followingreaction:

2(β-diketonate)Cu^((I))L_(n)→Cu^((O))+Cu^((II))(β-diketonate)₂+2L_(n)

The addition of a reaction reagent then reduces theCu^((II))(β-diketonate)₂ resulting in the deposition of additionalcopper. These reactions can be conducted sequentially or simultaneously.

In one embodiment the precursor is copper (I)(hexafluoroacetyl-acetonate)(2-butyne) (Cu(hfac)(2-butyne)), and thedeposition from Cu(hfac)(2-butyne) occurs via a two step-step reaction.The first step is shown below and occurs via a disproportionationreaction. This reaction does not require hydrogen and is thermallyinduced. At sufficiently high temperature, the reaction is nonselective,i.e., deposition occurs on all surfaces, whether or not seeded, andwhether or not metallic. The disproportionation reaction deposits a seedlayer of copper on any surface, including metals, metal oxides,nitrides, glasses, and polymers.

The second step is the reduction of the Cu(hfac)₂ formed during thedisproportionation reaction, which occurs via a hydrogen reduction. Thereaction occurs preferentially on an active metallic surface such asnickel, palladium, platinum, aluminum, copper, etc., such that thereduction of the Cu(hfac)₂ can then occur on the seed layer depositedthermally.

There are several advantages to this variation, which includeelimination of a separate step for deposition of the seed layer andhydrogen reduction of the thermal disproprotionation products that alsoallows for much higher atomic efficiency (approaching 100%) than thedisproportionation reaction alone, which results in an efficiency of atbest 50%. Variations of this process include conducting thedisproportionation and reduction reactions simultaneously orsequentially, and the use of mixtures of precursors including mixturesof (β-diketonate)Cu^((I))L_(n) and Cu^((II))(β-diketonate)₂.

To promote film purity, a preferred embodiment is to maintain thetemperature of the reactor and substrate at conditions that avoidthermal degradation of the liberated ligands and ligand products. Forexample it is known in the art that use of excessive reactiontemperatures (greater than ˜500K) for deposition of Cu fromCu^((II))(β-diketonate)₂ compounds can lead to the formation of carbonimpurities by thermal decomposition of the ligand and liganddecomposition intermediates (see, e.g., Girolami et al., J. Am. Chem.Soc., 115:1015-1024, 1993).

Solvents

Solvents useful as supercritical fluids are well known in the art andare sometimes referred to as dense gases (Sonntag et al., Introductionto Thermodynamics, Classical and Statistical, 2nd ed., John Wiley &Sons, 1982, p. 40). At temperatures and pressures above certain valuesfor a particular substance (defined as the critical temperature andcritical pressure, respectively), saturated liquid and saturated vaporstates are identical and the substance is referred to as a supercriticalfluid. Solvents that are supercritical fluids are less viscous thanliquid solvents by one to two orders of magnitude. In CFD, the lowviscosity of the supercritical solvent facilitates improved transport(relative to liquid solvents) of reagent to, and decomposition productsaway, from the incipient film. Furthermore, many reagents that would beuseful in chemical vapor deposition are insoluble or only slightlysoluble in various liquids and gases and thus cannot be used in standardCVD. However, the same reagents often exhibit increased solubility insupercritical solvents. Generally, a supercritical solvent can becomposed of a single solvent or a mixture of solvents, including forexample a small amount (<5 mol %) of a polar liquid co-solvent such asmethanol.

It is important that the reagents are sufficiently soluble in thesupercritical solvent to allow homogeneous transport of the reagents.Solubility in a supercritical solvent is generally proportional to thedensity of the supercritical solvent. Ideal conditions for CFD include asupercritical solvent density of at least 0.1 to 0.2 g/cm³ or a densitythat is at least one third of the critical density (the density of thefluid at the critical temperature and critical pressure).

Table 1 below lists some examples of solvents along with theirrespective critical properties. These solvents can be used by themselvesor in conjunction with other solvents to form the supercritical solventin CFD. Table 1 lists the critical temperature, critical pressure,critical volume, molecular weight, and critical density for each of thesolvents.

TABLE 1 Critical Properties of Selected Solvents T_(c) P_(c) V_(c)Molecular ρ_(c) Solvent (K) (atm) (cm/mol) Weight (g/cm³) CO₂ 304.2 72.894.0 44.01 0.47 C₂H₆ 305.4 48.2 148 30.07 0.20 C₃H₈ 369.8 41.9 203 44.100.22 n-C₄H₁₀ 425.2 37.5 255 58.12 0.23 n-C₅H₁₂ 469.6 33.3 304 72.15 0.24CH₃—O—CH₃ 400 53.0 178 46.07 0.26 CH₃CH₂OH 516.2 63.0 167 46.07 0.28 H₂O647.3 12.8 65.0 18.02 0.33 C₂F₆ 292.8 30.4 22.4 138.01 0.61

To describe conditions for different supercritical solvents, the terms“reduced temperature,” “reduced pressure,” and “reduced density” areused. Reduced temperature, with respect to a particular solvent, istemperature (measured in Kelvin) divided by the critical temperature(measured in Kelvin) of the particular solvent, with analogousdefinitions for pressure and density. For example, at 333 K and 150 atm,the density of CO₂ is 0.60 g/cm³; therefore, with respect to CO₂, thereduced temperature is 1.09, the reduced pressure is 2.06, and thereduced density is 1.28. Many of the properties of supercriticalsolvents are also exhibited by near-supercritical solvents, which refersto solvents having a reduced temperature and a reduced pressure bothgreater than 0.8, but not both greater than 1 (in which case the solventwould be supercritical). One set of suitable conditions for CFD includea reduced temperature of the supercritical or near-supercritical solventof between 0.8 and 1.6 and a critical temperature of the fluid of lessthan 150° C.

Carbon dioxide (CO₂) is a particularly good choice of solvent for CFD.Its critical temperature (31.1° C.) is close to ambient temperature andthus allows the use of moderate process temperatures (<80° C.). It isalso unreactive with most precursors used in CVD and is an ideal mediafor running reactions between gases and soluble liquids or solidsubstrates. Other suitable solvents include, for example, ethane orpropane, which may be more suitable than CO₂ in certain situations,e.g., when using precursors which can react with CO₂, such as complexesof low-valent metals containing strong electron-donating ligands (e.g.,phosphines).

Precursors and Reaction Mechanisms

Precursors are chosen so that they yield the desired material on thesubstrate surface following reaction with the reaction reagent.Materials can include metals (e.g., Cu, Pt, Pd, and Ti), elementalsemiconductors (e.g., Si, Ge, and C), compound semiconductors (e.g.,III-V semiconductors such as GaAs and InP, II-VI semiconductors such asCdS, and IV-VI semiconductors such as PbS), oxides (e.g., SiO₂ andTiO₂), or mixed metal or mixed metal oxides (e.g., a superconductingmixture such as Y—Ba—Cu—O). Organometallic compounds and metallo-organiccomplexes are an important source of metal-containing reagents and areparticularly useful as precursors for CFD. In contrast, most inorganicmetal-containing salts are ionic and relatively insoluble, even insupercritical fluids that include polar modifiers such as methanol.

Some examples of useful precursors for CFD include metallo-organiccomplexes containing the following classes of ligands: beta-diketonates(e.g., Cu(hfac)₂ or d(hfac)₂, where hfac is an abbreviation for1,1,1,5,5,5-hexafluoroacetylacetonate), alkyls (e.g., Zn(ethyl)₂ ordimethylcyclooctadiene platinum (CODPtMe₂)), allyls (e.g. bis(allyl)zincor W(η⁴-allyl)₄), dienes (e.g., CODPtMe₂), or metallocenes (e.g.,Ti(η⁵-C₅H₅)₂ or Ni(η⁵-C₅H₅)₂). For a list of additional potentialprecursors see, for example, M. J. Hampden-Smith and T. T. Kodas, Chem.Vap. Deposition, 1:8 (1995).

Precursor selection for CVD is limited to stable organometalliccompounds that exhibit high vapor pressure at temperatures below theirthermal decomposition temperature. This limits the number of potentialprecursors. CFD obviates the requirement of precursor volatility andreplaces it with a much less demanding requirement of precursorsolubility in a supercritical fluid.

Any reaction yielding the desired material from the precursor can beused in CFD. However, low process temperatures (e.g., less than 300,275, 250, 200, 150, or 100° C.) and relatively high fluid densities(e.g., greater than 0.1 to 0.2 g/cm³) in the vicinity of the substrateare important features of CFD. If the substrate temperature is too high,the density of the fluid in the vicinity of the substrate approaches thedensity of a gas, and the benefits of the solution-based process arelost. In addition, a high substrate temperature can promote deleteriousfragmentation and other side-reactions that lead to film contamination.Therefore a reaction reagent, rather than thermal activation, is used inCFD to initiate the reaction that yields the desired material from theprecursor.

For example, the reaction can involve reduction of the precursor (e.g.,by using H₂ or H₂S as a reducing agent), oxidation of the precursor(e.g., by using O₂ or N₂O as an oxidizing agent), or hydrolysis of theprecursor (i.e., adding H₂O). An example of an oxidation reaction in CFDis the use of O₂ (the reaction reagent) to oxidize a zirconiumbeta-diketonate (the precursor) to produce a metal thin film of ZrO₂. Anexample of a hydrolysis reaction in CFD is water (the reaction reagent)reacting with a metal alkoxide (the precursor), such as titaniumtetraisopropoxide (TTIP), to produce a metal oxide thin film, such asTiO₂. The reaction can also be initiated by optical radiation (e.g.,photolysis by ultraviolet light). In this case, photons from the opticalradiation are the reaction reagent.

Chemical selectivity at the substrate can be enhanced by a temperaturegradient established between the substrate and the supercriticalsolution. For example, a gradient of 40° C. to 250° C. or 80° C. to 150°C. can be beneficial. However, to maintain the benefits of CFD, thetemperature of the substrate measured in Kelvin divided by the averagetemperature of the supercritical solution measured in Kelvin istypically between 0.8 and 1.7.

In some cases, the supercritical fluid can participate in the reaction.For example, in a supercritical solution including N₂O as a solvent andmetal precursors such as organometallic compounds, N₂O can serve as anoxidizing agent for the metal precursors yielding metal oxides as thedesired material. In most cases, however, the solvent in thesupercritical fluid is chemically inert.

In the two-step process, the precursors and reaction mechanisms are asdescribed above. In the second step, a metal layer or film is depositedonto the seed layer, which serves as a catalyst for metallization. Thissecond step can be electroless or electrolytic deposition or a secondCFD step. Electroless plating baths contain metal salts, liquid reducingagents such as formaldehyde or hydrogen cyanide, stabilizers, and othercomponents. For example, components of copper plating baths arecommercially available and often include copper sulfate, formaldehyde,NaOH, chelating agents, and proprietary additives. An example is theM-Copper 85 plating system available from MacDermid Incorporated.Electrolytic plating solutions are also commercially available and oftencontain copper sulfate as the metal salt, and various additivesincluding sulfuric acid and hydrochloric acid.

EXAMPLES

The invention is further described in the following examples, which donot limit the scope of the invention described in the claims.

1. Platinum Film on a Silicon Wafer

A platinum metal film was deposited onto a silicon wafer by reduction ofdimethylcyclooctadiene platinum(II) (CODPtMe₂) with hydrogen gas in asupercritical CO₂ solution. Polished silicon test wafers (orientation:<100>, Boron doped type P, 450 microns thick), carbon dioxide (99.99%),and hydrogen gas (pre-purified grade) were commercially obtained andused without modification. CODPtMe₂ is useful because of its highplatinum content (58.5 wt. %), low toxicity of the ligands, and heptanesolubility, which is a good indicator of solubility in CO₂. Prior toCFD, solubility measurements of CODPtMe₂ in CO₂ were preformed in a viewcell. Results indicated that the solubility of the precursor was greaterthan 1% by weight at 40° C. and 100 bar and that no degradation ofprecursor was observed over a range of temperatures up to 80° C.

CODPtMe₂ was dissolved into supercritical CO₂ at 80° C. and 155 bar toproduce a 0.6% by weight precursor solution. The reaction vesselcontaining the precursor solution and silicon wafer was allowed toequilibrate for 2 hours. The precursor was then reduced by the additionof approximately 15×molar excess of H₂ gas. Reduction resulted in thedeposition of continuous, reflective Pt films on the silicon wafers.Scanning electron microscopy (SEM) analysis of the film revealedwell-defined 80-100 nm platinum crystals. The platinum film wasapproximately 1.3 microns thick and uniform as determined by SEManalysis of fracture cross-sections of the composite.

X-ray photoelectron spectroscopy (XPS) indicated that the film was freeof ligand-derived contamination. XPS was performed using a spectrometeremploying Mg Kα excitation (400 W 15.0 kV). FIG. 2A shows an XPS surveyspectrum taken after sputter cleaning with Ar⁺ ions to removeatmospheric contaminates. The spectrum gives the normalized number ofelectrons (in arbitrary units) ejected from a site in the film as afunction of the binding energy of that site. The small C_(1s) carbonpeak (284 eV) observed in the spectrum of the sputtered deposit is atthe detection limit of the instrument and could not be meaningfullyquantified by multiplex analysis. The continuity of the film wasconfirmed by the absence of Si_(2s) peaks at 153 eV (Si_(2p) peaks at102 and 103 eV would be obscured by the Pt_(5s) photoelectron line). Ptphotoelectron lines are observed at the following energies: 4f_(7/2)=73eV, 4f_(5/2)=76 eV, 4d₅=316 eV, 4d₃=333 eV, 4p₃=521 eV, 4p₁=610 eV, and4s=726 eV. For a reference on XPS see Christmann, Introduction toSurface Physical Chemistry; Springer-Verlag:New York (1991), Chap. 4.

2) Platinum Film on a Fluoropolymer Substrate

Platinum metal was deposited on a 0.95 gram sample of 0.9 mm thick sheetof polytetrafluoroethylene (PTFE) by reduction of CODPtMe₂ with H₂ gas,as generally described in Example 1. A 1.2% by weight solution ofCODPtMe₂ in CO₂ was equilibrated with the PTFE sample at 80° C. and 155bar for 4 hours. The precursor was then reduced by the addition of a15×molar excess of H₂ gas. Following deposition, the sample exhibited abright reflective coating. An SEM image of the surface of the sampleindicated the presence of relatively large platinum crystals. Platinumclusters were also observed in the bulk of the sample by transmissionelectron microscopy (TEM) analysis of interior sections of the sampleobtained by cryogenic microtomy.

3) Platinum Deposited Within Porous Aluminum Oxide

Anopore™ aluminum oxide (Al₂O₃) membranes having 200 nm straight poreswere obtained from Whatman International Ltd. (Maidstone, England) andused as a porous solid substrate. The pores are oriented perpendicularto the surface, are approximately hexagonally packed, and exhibit anarrow pore size distribution.

An 11.3 mg sample of an Al₂O₃ membrane was exposed to a 0.74 wt. %solution of CODPtMe₂ in CO₂ at 80° C. and 155 bar for two hours in asmall (ca. 3 ml) reaction vessel. CODPtMe₂ was then reduced by theaddition of H₂ gas, resulting in the deposition of platinum, as was donein Examples 1 and 2. After deposition, the surface of the membrane wasmetallic-gray in color. A sample of the metallized membrane was cast inepoxy and cross-sectioned by cryomicrotomy. TEM analysis of the sectionsindicated the presence of small Pt clusters distributed throughout thepores. Pt deposition within a second membrane at similar conditions(0.68 wt. % CODPtMe₂, 80° C., 155 bar, 2 hours, followed by reductionwith H₂) yielded similar results. Analysis of the second membrane by SEMrevealed small Pt clusters distributed throughout the pores.

4) Palladium Film on a Silicon Wafer

Palladium metal films were deposited by the hydrogenolysis of palladium(II) hexafluoroacetylacetonate (Pd(hfac)₂) in supercritical CO₂.Solubility of Pd(hfac)₂ in CO₂ was predicted based on the presence ofthe fluorinated ligands and confirmed by experiments in a view cell.With the exception of the precursor, Pd(hfac)₂, the procedure wassimilar to the one used in Example 1. A Si wafer in contact with a 0.62%by weight solution of Pd(hfac)₂ in CO₂ was equilibrated at 80° C. and155 bar for 2 hours. The precursor was then reduced by the addition of a15×molar excess of H₂ gas. The process produced a bright and reflectivePd film.

FIG. 2B shows an XPS survey spectrum taken after sputter cleaning withAr⁺ ions to remove atmospheric contaminates. There were no peaksdetected in the C_(1s) carbon region (280-290 eV) of the sputtereddeposit. The inset in FIG. 2B is an expansion of the XPS spectra in the280 eV to 300 eV region, which contains the C_(1s) region. Fluorinephotoelectron lines (Fl_(s)=686 eV) were not observed indicating nocontamination by the ligand or ligand-derived decomposition products. Pdphotoelectron lines are observed at the following binding energies (Mgsource): 4p₃=54 eV, 4s =88 eV, 3d₅=337 eV, 3d₃=342 eV, 3p₃=534eV,3p₁=561 eV, and 4s=673 eV. Auger lines are observed at 928 eV and 979eV. Additional experiments at similar conditions (e.g., 0.59 wt %Pd(hfac)₂, 80° C., 156 bar, 2 hours) yielded similar results.

5) Palladium Thin Film from Supercritical Ethane

A palladium thin film is deposited onto a silicon wafer by reduction ofpalladium(II) bis(2,2,7-trimethyl-3,5-octanedionate) (Pd(tod)₂) with H₂in supercritical ethane solvent. With the exception of the precursor,Pd(tod)₂, and the solvent, ethane, the procedure is similar to the onein Example 1. Temperature is set between 32° C. and 100° C., pressure isset between 75 and 500 bar, and the supercritical Pd(tod)₂ solutionconcentration is set between 0.01% and 1.0% by weight.

6) Copper Thin Film on a Silicon Wafer

A copper thin film was deposited onto a silicon wafer using a two-stepprocess. A patterned Si wafer was cleaned by sonication in acetone andmethanol and dried in a convection oven. A palladium seed layer was thendeposited onto the patterned Si wafer by hydrogen reduction of a 0.1 wt% solution of π-2-methylallyl(cyclopentadienyl) palladium(II) in CO₂ at60° C. and 138 bar using a procedure similar to that described inExamples 1 and 2. A conformal copper film was then deposited onto theseeded substrate by the hydrogen reduction of a 0.6 wt % solution ofbis(2,2,6,6-tetramethyl 3,5-heptanedionato)copper(II) (Cu(TMHD)₂) in CO₂solution at 120° C. and 276 bar. FIGS. 3A and 3B are SEM images of across-section of the fractured wafer after deposition of the film. It isevident that the deposited film is conformal to the wafer surface.Analysis of the film by x-ray diffraction revealed characteristicreflections for the (111) and (200) crystal planes of Cu.

7) Metal Sulfide Thin Film on a Silicon Wafer

A metal sulfide (e.g., CdS, PbS, and ZnS) film is deposited onto asilicon wafer by the reaction of the reaction reagent H₂S with asuitable alkyl, allyl, or beta-ketonate metal complex, for examplereduction of bis(allyl)zinc with H₂S to yield ZnS. The procedure issimilar to the one performed in Example 1 with the exception of theprecursor, bis(allyl)zinc, and the reaction reagent, H₂S. Thetemperature is set between 32° C. and 100° C., the pressure is setbetween 75 and 500 bar, and the supercritical bis(allyl)zinc solutionconcentration is set between 0.01 and 1.0 percent by weight.

8. Mixed Metal Thin Film of Y—Ba—Cu

A mixed metal film of Y—Ba—Cu is deposited onto a silicon wafer bydissolving metal beta-diketonates of Y, Ba, and Cu, such as Y(thd)₃,Ba(thd)₃, and Cu(thd)₃, into supercritical ethane to form a solutionwith a stoichiometric ratio of 1Y:2Ba:3Cu. H₂ gas is used as a reducingagent to decompose the precursors into elemental metal on the substratesurface. The procedure is similar to the one performed in Example 1 withthe exception of different precursors (i.e., metal beta-diketonates) anda different supercritical solvent (i.e., ethane). Temperature is setbetween 32° C. and 100° C., pressure is set between 75 and 500 bar, andthe supercritical solution concentration for each of the differentmetals is set between 0.01 and 1.0 percent by weight. Subsequent toforming the mixed metal film by CFD, the mixed metal film can beoxidized using standard techniques, for example by an oxygen plasma, togive a superconducting thin film of YBa₂Cu₃O_(7−x), (e.g., see Sieverset al U.S. Pat. No. 4,970,093).

9. Thin Film Metal Alloys of Platinum and Nickel

CFD was used to deposit thin films of Ni/Pt alloys of increasing Nicomposition spanning the composition range between the two elements.Bis(cyclopentadienyl)nickel (Cp₂Ni) and dimethyl(cyclooctadienyl)platinum (CODPtMe₂) were used as the metal precursors. Hydrogenolysis ofcosolutions of these compounds at 60° C. and 140 bar yielded the Ni/Ptalloys. The composition of the alloy was dictated by the stoichiometricratio of the precursors in supercritical CO₂ solution. X-ray diffraction(XRD) was used to determine the alloy composition in the films. FIG. 3Cshows the x-ray data for each of the films. In each case, the X-ray datadisplays a single diffraction peak between the x-ray diffraction angle2θ≈40° (corresponding to pure Pt (111)) and the x-ray diffraction angle2θ≈45° (corresponding to pure Ni (111)), corresponding to the formationof a homogeneous mixture with no evidence of pure regions of eitherparent metal. Furthermore, the regular shift in the 2θ values of thepeaks signifies the formation of a continuous series of alloys. Theinset in FIG. 3C shows alloy composition determined from the x-ray dataversus the feed composition for the metal precursors and reveals a near1:1 correspondence between alloy composition and fluid phasecomposition.

10. Conformal Films of Palladium on Patterned Si Wafers, Polyimide, andγ-Alumina

High purity palladium films were deposited onto patterned andunpatterned Si wafers, polyimide (Kapton®; DuPont), and γ-alumina by thereduction of organopalladium compounds in supercritical CO₂ solutionusing a batch process at temperatures between 40 and 80° C. andpressures between 100 and 140 bar. Table 2 below lists the filmsproduced.

TABLE 2 Complex T (° C.) P (bar) OM wt. %^(a) SubstrateAdhesion^(b)/Comments Pd(hfac)₂ 60 103 0.37 Kapton Bright metallic film;fair adhesion Pd(hfac)₂ 60 124 0.28 Kapton Bright metallic film; fairadhesion Pd(hfac)₂ 80 138 0.29 Kapton Metallic film; fluid phasenucleation; poor adhesion Pd(hfac)₂ 60 124 0.30 Ti/W-Si^(c) Metallicfilm; poor adhesion (π³-C₃H₅)Pd(acac) 40 103 0.38 Silicon Brightmetallic film; poor adhesion (π³-C₃H₅)Pd(acac) 40 103 0.25 KaptonNonuniform bright metallic film (π³-C₃H₅)Pd(acac) 60 103 0.24 KaptonBright metallic film; good adhesion (π³-C₃H₅)Pd(acac)^(d) 40 138 0.18Kapton Metallic film; good adhesion (π³-C₃H₅)Pd(acac)^(d) 40 138 0.18γ-Alumina Dull gray metallic film; good adhesion (π³-C₃H₅)Pd(acac)^(d)40 138 0.36 Kapton Dark gray metallic (π³-C₃H₅)Pd(acac)^(d) 40 138 0.36γ-Alumina Dark gray metallic CpPd(π³-C₄H₇) 60 103 0.41 Kapton Brightmetallic film; poor adhesion; fluid phase nucleation CpPd(π³-C₄H₇) 60103 0.21 Kapton Metallic film CpPd(π³-C₄H₇) 60 138 0.21 Kapton Brightmetallic film; fluid phase nucleation CpPd(π³-C₄H₇) 60 138 0.24 KaptonBright metallic film; fluid phase nucleation CpPd(π³-C₄H₇) 60 138 0.26Patterned Bright metallic film Silicon

In Table 2, superscript “a” denotes weight percent calculated by weightof organometallic (OM) divided by weight of CO₂ added. Superscript “b”indicates that an explanation of adhesion is described below.Superscript “c” denotes that the Si wafer was sputtered with ˜7 nm of88% Ti/12% W. Superscript “d” denotes that the depositions wereperformed thermally without hydrogen addition. The precursor synthesisand deposition are described in greater detail below.

Allylpalladium chloride dimer, palladium (II) chloride, andcyclopentadienylthallium (99% sublimed) were purchased from StremChemicals. Sodium acetylacetonate monohydrate, 3-chloro-2-methylpropene(2-methylallyl chloride), and palladium(II) hexafluoroacetylacetonate,Pd(hfac)₂, were purchased from Aldrich Chemical Co. Each was used asreceived. Precursor synthesis was carried out under inert atmospheres ofpurified argon using standard Schlenk techniques. Transfer of airsensitive reagents during synthesis and loading of high-pressurereaction vessels with palladium precursors was conducted inside a glovebox that had been purged for 30 minutes with pre-purified nitrogen. Allsolvents used for air or moisture sensitive reactions were dried oversodium and benzophenone ketyl, then distilled under an atmosphere ofpurified argon.

π-allylpalladium acetylacetonate, (π³-C₃H₅)Pd(acac), was synthesizedusing standard procedures immediately prior to use. See, e.g., S.Imamura et al. in J. Bulletin of the Chemical Society of Japan(42:805-808, 1968). (π³-C₃H₅)Pd(acac) was found to be pure by protonNMR. ¹H NMR: 200 MHz in CDCl₃, 2.01 ppm (s, 6H, CH₃ acac), 2.90 ppm (d,J_(H-H)=11.9 Hz, 2H, anti-H of CH₂), 3.88 ppm (d, J_(H-H)=6.8 Hz, 2H,syn-H of CH₂), 5.39 ppm (s, 1H, C—H acac), 5.57 ppm (triplet oftriplets, 1H, C—H allyl). 2-Methylallyl palladium chloride dimer wasprepared according to standard literature procedures and stored coldunder argon. See, e.g., M. Sakakibara et al. in Y. ChemicalCommunications (1969-1970, 1969). [(π³-C₄H₇)PdCl]₂ was found to be pureby proton NMR. ¹H NMR: 200 MHz in CDCl₃, 2.15 ppm (s, 3H, CH₃), 2.89 ppm(s, 2H, anti-H of CH₂), 3.87 ppm (s, 2H, syn-H of CH₂).

2-Methylallylpalladium cyclopentadienylide, CpPd(π³-C₄H₇), wassynthesized from [(π³-C₄H₇)PdCl]₂ and cyclopentadienythallium. In astandard preparation, 2.63 g (6.676 mmol) of [(π³-C₄H₇)PdCl]₂ and twoequivalents (3.60 g, 13.35 mmol) of cyclopentadienythallium were sealedtogether in a Schlenk flask under an atmosphere of nitrogen. The flaskwas then transferred to a Schlenk line where its nitrogen atmosphere wasreplaced with purified argon. The flask was cooled to −78° C. and 70 mlof tetrahydrofuran, previously dried from sodium and benzophenone ketyl,was condensed onto the reagents. With constant stirring, the reactionwas warmed overnight to room temperature whereupon the mixture consistedof a dark red THF solution and an off-white precipitate. The solutionwas filtered under a stream of argon to remove the insoluble thalliumchloride. The clear red filtrate was reduced in volume under vacuumuntil a dark-red crystalline material remained. This material was easilysublimed (60° C. at 0.2 torr) to remove trace impurities, yielding longdark-red needles of CpPd(π³-C₄H₇). Yield 2.12 g, 70%. The product wassublimed again prior to each deposition reaction and found to be pure byproton NMR. ¹H NMR: 200 MHz in d₆-benzene, 1.69 ppm (s, 3H, CH₃), 2.12ppm (s, 2H, anti-H of CH₂), 3.39 ppm (s, 2H, syn-H of CH₂), 5.86 ppm (s,5H; C₅H₅).

Boron-doped, P-type silicon test wafers with a <100> orientation wereobtained from International Wafer Service. Etched Si test wafers wereobtained from Novellus Systems. All wafers were cleaned by immersion inboiling deionized water, followed by rinsing and sonication in acetoneor methyl alcohol for 20 minutes. The wafers were then dried overnightin air at 100° C. Sheets of Kapton® polyimide (Dupont) of 50 μmthickness were cleaned using a similar procedure. γ-alumina substrateswere prepared from porous α-alumina disks prepared by the compression ofα-alumina powder in a mechanical press followed by heating at 1200° C.for a period of 30 hours. The resulting pressed disks were then coatedwith a thin layer of γ-alumina by the sol-gel process using a 1-Mboehmite sol doped with polyvinyl alcohol. Carbon dioxide (Colemangrade) and hydrogen (High Purity grade) were obtained fromMerriam-Graves and used as received.

All depositions from CO₂ were conducted in high-pressure stainless steelreactors (High Pressure Equipment Company) with an approximate volume of17 ml. A single substrate (˜1 by 4 cm samples of silicon wafer or ˜1 by7 cm samples Kapton polyimide) and a known amount (10-40 mg) ofprecursor were added to the vessel. Pd(hfac)₂ was loaded at ambientconditions. The precursor (π³-C₃H₅)Pd(acac) decomposes slowly in air atroom temperature and was therefore loaded under a stream of argon orinside a nitrogen glove box. CpPd(π³-C₄H₇) is stable for short periodsof time in the atmosphere, but was loaded in the dry box as aprecaution. The vessels were sealed with a plug at one end and ahigh-pressure needle valve at the other. The vessels were purged withCO₂, weighed, and placed in a circulating constant temperature bath(40-80° C.) and allowed to equilibrate to the desired temperature.Carbon dioxide was then added to the desired pressure via ahigh-pressure manifold or a high-pressure syringe pump (Isco, Inc. Model260D) that was maintained at the same temperature as the constanttemperature bath. The vessels were mixed with a vortex mixer or bycyclic inversion and weighed to determine the mass of CO₂ added. Thevessels were then returned to the constant temperature bath for 1 hourto ensure complete dissolution of the precursor.

Film deposition was initiated by reduction of the precursor eitherthermally or upon reaction with hydrogen using the CFD methods. Forhydrogenolysis, a 60-100 fold molar excess of H₂ was transferred to thereaction vessel via a small pressurized manifold (3.6 ml), consisting ofa section of high pressure tubing capped at both ends with high pressureneedle valves and equipped with a pressure gauge. The quantity ofhydrogen admitted into the vessel was calculated by the pressure-dropmeasured in the manifold. The rate of hydrogen addition was regulatedusing a needle valve at approximately 200 μmols per minute. Afterhydrogen addition, the reactions were allowed to stand for a period of1-2 hours, although kinetic experiments indicate the reaction iscomplete in less than 2 minutes for CpPd(π³-C₄H₇). View cell experimentssuggested a similar reaction period for Pd(hfac)₂. The vessels were thendepressurized and the effluent directed through an activated charcoalbed. In some cases, the reaction by-products contained in the CO₂mixture were first collected for analysis by venting the vessel throughan appropriate solvent.

Solubility of the precursors in CO₂ at the deposition conditions wasverified optically using a variable volume view cell. X-rayphotoelectron spectroscopy (XPS) was conducted using a Perkin-ElmerPhysical Electronics 5100 spectrometer with Mg Kα excitation (400 W 15.0kV). To ensure representative analysis, a sampling area of 4×10 mm wasemployed. The films were cleaned by Ar+ sputtering to remove atmosphericcontaminants until the measured atomic concentrations in consecutiveanalyses were invariant. Field emission scanning electron microscopy(FE-SEM) was performed using a JEOL 6400 FXV SEM and an acceleratingvoltage of 5-10 kV. Analysis of cross-sections of Pd films on polyimidewas conducted after embedding in epoxy and subsequent cryo-microtomy.Wide-angle x-ray diffraction (WAXD) was performed on a Phillips X'PertPW 3040 with Cu K_(α) radiation.

As indicated in Table 2, palladium was deposited onto the substrates byhydrogenolysis of the precursor or by thermal decomposition for some ofthe (π³-C₃H₅)Pd(acac) precursors at 40-80° C. Pressures were between 100and 140 bar and precursor concentrations in CO₂ ranged from 0.2 to 0.6wt. %. A 60-100 fold molar excess of H₂ was used for each hydrogenation.The resulting films were analyzed by x-ray photoelectron spectroscopy(XPS), scanning electron microscopy (SEM) and wide-angle x-raydiffraction (WAXD). The XPS survey of the palladium films indicated thatsubsequent Ar⁺ sputtering removed atmospheric contaminants, with anycarbon impurities being below the detection limits of the instrument.Fluorine contamination (600 eV and 688 eV) was not observed in any ofthe films deposited from Pd(hfac)₂ suggesting that ligand-derivedcontamination does not occur.

The morphology of palladium films grown on polyimide was studied usingSEM. The film deposited from Pd(hfac)₂ contains grains of approximately10 nm in size. The film deposited from CpPd(π³-C₄H₇) consists ofwell-defined palladium grains of ˜30-70 nm in diameter. This near orderof magnitude increase in grain size using CpPd(π³-C₄H₇) as the precursorrelative to films deposited from Pd(hfac)₂ was typical under thesedeposition conditions. SEM analysis of the polymer/Pd filmcross-sections revealed Pd was not deposited within the substrate. Thepolycrystalline nature of the metal film was verified by WAXD, whichrevealed only the broad Pd 111, 200, and 220 reflections in locationsconsistent with the small primary grain sizes observed by SEM. Filmsdeposited onto polyimide from Pd(hfac)₂ and CpPd(π³-C₄H₇) were ˜100 nmto 200 nm thick, as calculated by mass gain and confirmed by analysis offilm cross-sections by SEM. The initial loading of the precursordictates film thickness in these batch experiments. Thicker films can begrown by sequential batch deposition cycles. While the solubility ofboth precursors readily exceeds 1 wt. % at 60° C. and 138 bar, operationat concentrations above 1 wt. % may lead to gas phase nucleation anddeposition of Pd particulates. Gas phase nucleation was also observedupon rapid addition of excess H₂ at all precursor concentrations and wasmitigated by limiting the rate of H₂ addition to less than 200 μmols/minusing a needle valve.

(π³-C₃H₅)Pd(acac) decomposes relatively slowly at room temperature (forthe purposes of these methods) and was chosen as a candidate precursorfor low temperature thermal decompositions from CO₂ solution.Thermolysis of (π³-C₃H₅)Pd(acac) in neat CO₂ at 40° C., yielded dark,poorly reflective films. XPS analysis confirmed extensive carboncontamination (˜8 wt. %). SEM analysis revealed inhomogeneous films withdistinct regions of large globular metallic clusters greater than 100 nmin diameter and regions containing particles with a broad sizedistribution. Carbon contamination was reduced by the addition ofhydrogen using the CFD methods. XPS analysis revealed films deposited inthis manner are free of oxygen contamination and carbon contamination isless than 5%.

Reduction of deep red solutions of CpPd(π³-C₄H₇) in supercritical CO₂within a variable volume view cell proceeded immediately upon additionof H₂ as evidenced by the immediate loss of all color in the cell andthe formation of a metallic film on the quartz window. Nuclear magneticresonance spectroscopy (NMR) was used to analyze the products ofCpPd(π³-C₄H₇) hydrogenolysis in CO₂. Samples of the reaction productswere obtained by passing the effluent gases from completed depositionsin a standard reaction vessel through either deuterated acetone orchloroform (3-4 grams). Complete reduction of all ligands under theseconditions was confirmed by the observation of a singlet at 1.5 ppm anda doublet at 0.87 ppm corresponding to the protons in cyclopentane andthe methyl groups in isobutane (2-methylpropane), respectively. Theremaining proton of isobutane [(CH₃)₃—C—H] overlapped with a signalattributed to the solvent. An additional signal, appearing as a singlet,was observed at 2.86 ppm and is attributed to dissolved hydrogen. Thiswas confirmed by analysis of H₂-saturated d-acetone solutions. Noadditional proton signals were observed, confirming complete reductionof all double bonds of the cyclopentadienyl and allyl ligands to theiralkane analogs.

¹H NMR was also used to confirm that the reduction of CpPd(π³-C₄H₇) inCO₂ upon H₂ addition is rapid at the conditions used in thesedepositions. Hydrogenation was monitored by removing small aliquots ofthe reaction media at regular time intervals following introduction ofH₂ to the solution of CO₂ and CpPd(π³-C₄H₇). NMR analysis of theeffluent indicated that the precursor was completely consumed within thefirst two minutes of the reaction: all resonances assigned toCpPd(π³-C₄H₇) are absent and replaced by the signals from cyclopentane,isobutane, hydrogen, and only trace amounts of unidentified, unsaturatedspecies. Analysis of a second aliquot, taken from the reaction 22minutes after H₂ addition, indicated complete hydrogenation of thetrace, unsaturated products. The third, and all subsequent aliquotstaken after 42 minutes, revealed the peaks are invariant with additionalreaction time. These results were consistent with observations in theview cell that suggest an induction period for the reaction is notpresent.

The products from the hydrogenolysis of (π³-C₃H₅)Pd(acac) at 40° C. and103 bar and thermolysis at 40° C. and 103 bar were also analyzed.Visible at 0.810 ppm and 1.51 ppm are the predicted proton resonancescharacteristic of propane, resulting from the hydrogenolysis of thebound allyl group, which appear as the expected triplet and multipletrespectively. An additional set of resonances was also observed at 1.05ppm (d), 1.94 ppm (mult.), 2.95 ppm (br. s), and 4.06 (mult.) andmatches those of 2,4-pentanediol (C₅H₁₂O₂) (Scheme 2). A third,unidentified minor product was observed between 1.6 and 1.9 ppm. Thisspecies is the only ligand product observed when deposition wasperformed thermally, in particular, the proton resonances correspondingto propane and 2,4-pentanediol evident upon the hydrogenolysis of(π³-C₃H₅)Pd(acac) were not observed when this precursor was decomposedthermally. The ligand product is presumed to be the result ofdecomposition via reductive elimination prior to the introduction of H₂.Finally, unreacted (π³-C₃H₅)Pd(acac) or the presence of 2,4-pentanedione(C₅H₈O₂) was not detected in either method of palladium deposition.

Conformal deposition of continuous, pure palladium films onto silicontest wafers with features as small as 0.1 μm wide by 1 μm high wasobserved using the precursor CpPd(π³-C₄H₇) via batch CFD at 138 bar and60° C. Cross-sections of the films were prepared for SEM analysis byscoring the reverse of the wafers and then fracturing carefully by hand.Images of representative regions are shown in FIGS. 4A-D. FIGS. 4A and4B illustrate the smallest of these features with dimensions between100-120 nm in width by 1 μm deep. FIGS. 4C and 4D illustrate largerfeatures with dimensions of 0.7 μm wide (4 c) and 0.3 μm wide (4 d) by 1μm deep. Palladium deposition was found to be conformal to all surfacefeatures forming a thin layer of metal that exactly matched thetopography of the wafer, including the corners at the bottom of etchedfeatures. Conformal coverage was maintained in the high aspect ratiofeatures (0.1 μm wide by 1 μm deep) in which the opposing layers ofpalladium metal on either side of the feature nearly join.

The small space in FIG. 4B at the bottom of the feature is due tolifting of the metal film from the SiO₂ surface during wafer fracture.Within the larger features, the thickness of the film is approximately100 nm and is uniform along the sides and bottom of each trench. Noexcess buildup of palladium was observed on the top of the features,which suggests that any seams in the nearly filled narrow trenches couldbe filled completely by subsequent batch depositions or by deposition ina continuous flow reactor.

FIGS. 5A to 5D show additional SEM micrographs of conformal trenchfilling by CFD. The deposition of conformal thin palladium films, whichare evenly distributed over all areas of the topologically complexsurface, could also serve as a seed layer for subsequent electrolyticdeposition of other metals.

11. Two-step Process of Applying a Palladium Seed Layer and CopperPlating

One two-step process involves a first CFD step to deposit a palladiumseed layer on a silicon wafer, followed by an electroless copper. bathto plate the seeded substrate.

Silicon Wafer Preparation: Patterned silicon wafers measuring roughly 1cm×1.5cm were cleaned as follows: 1) Rinse under flowingtetrahydrofuran; 2) Rinse under flowing Reverse Osmosis (R.O.) H2O; 3)Boil˜1 hour in 37% HCl; 4) Boil˜2 hours in R.O. H₂O; and 5) Sonicate inwarm THF for 30 minutes. Silicon substrates were then transferred to aconvection oven at 325° F. for roughly one hour prior to transfer to aN₂ atmosphere glovebox.

Palladium Seed Layer Deposition: Under a N₂ atmosphere, wafers wereloaded into a 20 ml high pressure reactor along with 8 mgp-methylallylcyclo-pentadienyl Palladium (II) [sublimed<48 hours prior]and the vessel was sealed before removal from the inert atmosphere. Thereactor was pressurized to 2000 psi with CO₂ at 60° C. and soaked for 20minutes. H₂ was injected into the reactor via a pressure drop from 3000to 2600 psi on a 3.8 ml high-pressure manifold over roughly 3 minutes.The vessel was allowed to soak for an additional 60 minutes prior todepressurization through an activated charcoal filter. Substrates weretransferred directly from the reactor vessel to a prepared MacDermidM-Copper 85 Electroless copper bath.

As shown in FIG. 6A, palladium clusters formed on the substrate. FIG. 6Bshows a close-up view, in which the seed clusters are present within atrench only 140 nm wide. The clusters are between about 17.0 and 24 nmin diameter.

Electroless Copper Bath Preparation and Copper Deposition: The bath usedwas a MacDermid, Inc., M-Copper 85 Electroless Copper Bath (Product CodeNumber: 12440). The electroless copper bath was prepared on a 200 mltotal bath volume scale according to manufacturer's instructions and asfollows. 165 ml of R.O. H₂O was added to a 250 ml Pyrex recrystallizingdish. The water was stirred with a magnetic TEFLON® stir bar andcontinuously sparged with N₂ through a 1 μM pore size fritted glassfilter. The water was heated to 45° C.±˜0.5° C. with a heating mantleprior to addition of other bath constituents. Bath components were addedin the following sequence with rapid agitation and N₂ sparging:

Bath Component Volume Solution B-EDTA Complex 20 ml Solution A-CuSO4(aq)[300 g/L] 8.0 ml Solution D (provided by manufacturer) 3.0 ml Solution G(provided by manufacturer) 0.2 ml 37% Formaldehyde 0.5 ml

Palladium seeded Si substrates were transferred directly from thehigh-pressure vessel to the electroless bath with a total atmosphericexposure of less than one minute. Substrates were immersed in theplating solution for 7, 10, and 20 minutes. Each of the substrates wasremoved in sequence from the plating solution, washed under flowing R.O.H₂O and directly transferred to a vacuum dessicator. All films display abright, shiny metallic copper film that did not oxidize in the vacuumdessicator. As shown in FIGS. 7A and 7B copper deposition proceedsreadily within, and fills high aspect ratio trenches in, the etched Siwafers. FIG. 7A shows Cu deposition within trenches that are 165 nm wideby 1100 nm deep. FIG. 7B shows Cu deposition within a trench that is 235nm wide by 600 nm deep.

12. Two-step Process of Applying a Platinum Seed Layer and CopperPlating

Another two-step process involves a first CFD step to deposit apalladium seed layer on a silicon wafer, followed by an electrolesscopper bath to plate the seeded substrate.

Silicon Wafer Preparation: Patterned silicon wafers measuring roughly 1cm×1.5 cm are cleaned as described in Example 11. Silicon substrates arethen transferred to a convection oven at 325° F. for roughly 1 hour. Theclean and dry silicon wafers are transferred to a hot nitric acid bath(150 ml concentrated nitric acid/50 ml R.O. H₂O). The substrates aresoaked in the acid bath for ˜4 hours, rinsed with flowing R.O. H₂O, andthen transferred to a boiling R.O. H₂O bath for an additional 2 hours.Substrates are immediately transferred from the water bath to test tubesfor storage in a vacuum dessicator until use.

Silicon Wafer Functionalization: Silicon substrates oxidized aspreviously mentioned are covered with roughly 20 ml of a wet toluenesolution. The toluene solution is prepared by addition of 1.00 ml R.O.H₂O to a 500 ml of stock toluene. The test tubes are capped withaluminum foil and test tube vial screw caps. Transfer of wet toluene andsoaking is performed with exposure to ambient conditions.

After ˜24 hours of soaking in the wet toluene solution, the Si sampletest tubes are transferred to a N₂ atmosphere glovebox. Substrates aretransferred to clean, dry 20 ml screw cap test tubes where 15.0 ml ofdistilled, dry toluene is added. To each test tube, 3.75 ml of4-[2-(trichlorosilyl)ethyl]pyridine is added to produce a solution. Thesame volumetric ratio of 4 parts dry toluene: 1 part coupling agent isused for various silane coupling agents. The optimal ratio and reactiontimes are highly dependent upon the coupling agent reactivities. Thecoupling agents can include: 2-[trimethoxy-silylethyl]pyridine; 3,3,3-trifluoropropyltrichlorosilane; 3-iodopropyltrimethoxysilane; and4-(3-triethoxysilylpropyl)-4,5-dihydroimidazole chlorotrimethoxysilane.

Reaction test tubes are capped with aluminum foil and test tube screwcaps. A soaking period of between 24 and 72 hours is permitted for thereaction of the coupling agent with the substrate. After this time andstill under an inert atmosphere, the substrates are removed from thecoupling agent solution and washed with two 20 ml volumes of stocktoluene. Samples are then removed from the glove box under a stocktoluene solution in capped test tubes and are washed with two 20 mlvolumes of anhydrous methanol. The functionalized wafers are thenimmediately transferred to a vacuum dessicator where they were storeduntil use.

The purpose of these functional groups introduced by silanationchemistry is to bind to the organometallic compound prior to reductionand to impart control of cluster size during reduction either in thepresence of the supercritical fluid or after extraction anddepressurization.

Platinum Seed Layer Deposition and Extraction: Under a N₂ atmosphere,wafers are loaded into a 20 ml high pressure reactor along with 7 mgDimethylcyclo-octadiene Platinum (II) and the vessel is sealed beforeremoval from the inert atmosphere. The reactor is pressurized to 2000psi with CO₂ at 40° C. and remains closed for 60 minutes. The wafers inthe reactor are then extracted with 60.0 ml of CO₂ over ˜2 hours at aflow rate between 0.30 and 0.80 ml/minute. H₂ is injected into thereactor via a pressure drop from 3000 to 2600 psi on a 3.8 mlhigh-pressure manifold over roughly 1.5 minutes. Substrates in thevessel are allowed to soak for an additional 60 minutes prior todepressurization through an activated charcoal filter. Substrates aretransferred directly from the reactor vessel to a vacuum dessicator forstorage until use.

Electroless Copper Bath Preparation and Copper Deposition: The same bathas used in Example 11 is used here. The electroless copper bath isprepared on a 400 ml total bath volume scale according to themanufacturer's instructions and as follows. 325.5 ml of R.O. H₂O isadded to a 250 ml Pyrex recrystallizing dish. The water is stirred witha magnetic TEFLON® stir bar and continuously sparged with N₂ through a 1μM pore size fritted glass filter. The water is heated to 35° C.±˜2.0°C. with a heating mantle prior to addition of other bath constituents.Bath components are added in the following sequence with rapid agitationand N₂ sparging:

Bath Component Volume Solution B-EDTA Complex 40 ml Solution A-CuSO4(aq)[300 g/L] 16 ml Solution D (provided by manufacturer) 12 ml Solution G(provided by manufacturer) 0.8 ml 37% Formaldehyde 2.0 ml

Platinum seeded substrates functionalized with3,3,3-trifluoropropyl-trichlorosilane and4-[2-(trichlorosilyl)ethyl]pyridine or other silane coupling agents inthe manner outlined above can be examined using a JEOL 6320 fieldemission SEM at 5 kV accelerating voltage.

13. Copper Deposition Onto a Palladium Seeded Silicon Wafer

A thin copper film was deposited onto a palladium seeded silicon waferin a cold wall reactor using Cu(II) bishexafluoroacetylacetonate(Cu(hfac)₂) at substrate temperatures of 200° C. The Pd was seeded ontothe Si wafer by CFD as described above; however, any known standardtechniques of applying a seed layer, such as sputtering, can be used.

First, the Cu(hfac)₂ precursor was dissolved into supercritical CO₂ at60° C. and 124 bar. The concentration of the precursor was 0.6% byweight. The reaction vessel was allowed to equilibrate for 1 hour at 60°C. and 124 bar. The substrate was then heated to the desiredtemperature, 200° C. The final pressure was 193 bar. Hydrogen (93 foldmolar excess) was then added to the mixture. After three hours, thevessel was cleaned via a CO₂ extraction to remove by-products, drained,and the substrate removed. The resulting film was a highly reflectivecopper colored film.

An SEM of the cross-section of the film deposited is shown in FIG. 8A.The SEM shows conformal coverage and uniform filling of deep trenches.An XPS survey spectrum of the film is shown in FIG. 8B. The spectrumshows binding energies characteristic of copper. The copper issubstantially free of carbon as evidenced by the carbon 1s bindingenergy at about 284 eV. The density of the CO₂ near the substratesurface at 200° C. and 193 bar is known to be about 0.25 g/ml. The bulktemperature of the fluid is lower such that the bulk density of thefluid was approximately 0.47 g/ml based on the initial CO₂ loading ofthe vessel.

Under identical conditions, no deposition took place on an unseededsilicon wafer. Also, no deposition took place in the absence ofhydrogen. In addition, flourine NMR revealed that the only by-productsof the reaction were 2,4-hexafluouropentane-dione and trace residualprecursor.

14. Copper Deposition Onto a Palladium Seeded Silicon Wafer

A thin copper film was deposited onto a Pd seeded Si wafer wafer in acold wall reactor using Cu(I) (hexafluouroacetylacetonate)(2-butyne) atsubstrate temperatures between 175° C. and 200° C. As in Example 13, thePd was seeded onto the wafer by CFD as described above.

The precursor was dissolved in supercritical CO₂ at 60° C. and allowedto equilibrate for 1 hour before the substrate was heated to 200° C.Hydrogen (33 fold molar excess) was then added to the mixture. Theconcentration of the precursor was 0.6% by weight. The final pressure ofthe cold wall reactor vessel was 200 bar. The result of the depositionwas a highly reflective copper film. An SEM of the cross-section of thefilm is shown in FIG. 9A. This SEM shows complete filling of narrowtrenches with copper metal. FIG. 9B shows an XPS survey spectrum of thecopper film. The spectrum shows the binding energies characteristic ofcopper. The copper is substantially free of carbon as evidenced by thecarbon 1s binding energy at about 284 eV. The density of the carbondioxide near the substrate surface at 200° C. and 200 bar is known to be0.26 g/ml. The bulk temperature of the fluid is lower such that the bulkdensity of the fluid was approximately 0.47 g/ml.

Under these conditions, little to no deposition took place on anunseeded silicon wafer. In the absence of hydrogen, light depositiontook place on seeded or metallic surfaces. However, the addition ofhydrogen greatly increased the amount of deposited copper metal on thesubstrate. This is important because it indicates a higher yield of thecopper being deposited, suggesting that reduction of the Cu(II) reactionproduct has occurred.

15. Copper Deposition Onto a Unseeded Silicon Wafer

A thin copper film was deposited using Cu(I)(hexafluoroacetylacetonate)(2-butyne) at substrate temperatures above225° C. in a cold-wall reactor. The precursor was dissolved insupercritical carbon dioxide at 60° C. and allowed to equilibrate for 1hour before the substrate was heated to 225° C. Hydrogen (49 foldexcess) was then added to the mixture. The concentration of theprecursor was 0.48% by weight. The hydrogen/precursor ratio was 48.82.The final pressure of the vessel was 200 bar. The result of thedeposition was a highly reflective copper film. The density of thecarbon dioxide near the substrate surface at 225° C. and 200 bar was0.23 g/ml. The bulk temperature of the fluid is lower such that the bulkdensity of the fluid was approximately 0.47 g/ml.

At temperatures below 200° C., as in Example 14, no deposition tookplace on unseeded surfaces. However, at a temperature of 225° C.,deposition occurred on all surfaces, seeded, unseeded, metallic, orpolymers such as Kapton® polyimide. This indicates that at a temperaturebetween about 200° C. and 225° C., and certainly at or above 225° C.,the deposition becomes non-selective and a seed layer is no longerrequired.

In the absence of hydrogen, light deposition of copper occurred.However, the addition of hydrogen greatly increased the amount of metaldeposited, which indicates an increase in efficiency as described above.

16. Nickel Deposition Onto Glass and Silicon Substrates

Ni was deposited from CO₂ solution using bis(cyclopentadienyl)nickel(NiCp₂) as the metal precursor. In the absence of a catalytic surface,NiCp₂ is relatively stable in CO₂ solution at 200 bar and 60° C. uponthe addition of hydrogen, but the reduction proceeds readily at the sameconditions over Ni, Pd, or Pt films. Over nickel, the reaction yieldstwo equivalents of cyclopentane and one equivalent of Ni metal, which isincorporated into, and expands, the incipient metal deposit. Plating onnon-catalytic surfaces such as polymers was accomplished by seeding thedeposition using two strategies. In one embodiment, co-solutions ofNiCp₂ and an easily reduced organopalladium compound such as2-methylallyl(cyclopentadienyl)palladium(II) [CpPd(C4H7)] were preparedin stoichiometric ratios ranging from 0.005:1 to 0.1:1 (Pd:Ni). Forexample, films were co-deposited from a CO₂ solution of NiCp₂ (0.2 wt.%) at 60° C. and 140 bar and CpPd(C₄H₇) (0.02 wt. %) onto glass. XPSanalysis revealed Ni peaks at their appropriate binding energies andinsignificant levels of carbon contamination.

In co-reductions with NiCp₂, rapid hydrogenolysis of CpPd(C₄H₇) yieldscatalytic clusters that serve as nucleation sites for the reduction ofNiCp₂. Once the incipient metal surface is formed, growth of the Ni filmproceeds readily. This was confirmed by composition analysis using x-rayphotoelectron spectroscopy (XPS) depth profiling.

The second embodiment involves activation of the substrate by thedeposition of an initial seed layer of palladium clusters using CFD.This approach confines the subsequent deposition of pure nickel to thePd-activated surfaces only, and thus affords opportunities for selectivefilm growth through control of the spatial distribution of the seedlayer. Ni films prepared by hydrogen reduction of 0.2% solutions ofNiCp₂ in CO₂ at 140 bar and 60° C. The metal coatings are reflective,continuous, and free of contamination as determined by XPS.

Deposition of Ni onto patterned Si wafers reveals that the high reagentconcentrations and favorable fluid-phase transport properties inherentto CFD facilitate conformal deposition. FIGS. 10A to D show SEMmicrographs of films deposited within trenches of etched silicon wafersfrom solutions of NiCp₂ using either a batch or a continuous-feeddeposition process. A batch process was used to completely fill trenchesas narrow as 83 nm with aspect ratios of greater than 10 (FIG. 10A). Theuniform filling of the features implies excellent conformality of the Pdseed layer within these deep trenches. Faithful reproduction of thewafer topology and Ni film continuity and cohesion are evident in FIG.10B, which shows a 100 nm thick Ni film that has delaminated andseparated from the etched Si substrate containing micron-wide trenchesduring the fracturing process employed for sample preparation.

In the batch process (FIGS. 10A and B), film thickness is dictated bythe initial precursor loading (0.2 wt. %). The deposited films can begrown to virtually any thickness using a flow reactor in which thereactants are continuously fed to the deposition chamber and theby-products removed via an effluent stream. FIG. 10C shows a seamless,2.5 micron- thick film deposited in a continuous-flow reactor at 60° C.and 180 bar completely filling 450 nm-wide trenches in the Si wafer.FIG. 10D shows a Ni film grown in the flow reactor on a substratecontaining 75 nm trenches. In this example, two of the trenches were notetched completely during fabrication of the test wafer, leaving featuresas narrow as 45 nm (far right), which are filled upon Ni deposition.

OTHER EMBODIMENTS

It is to be understood that while the invention has been described inconjunction with the detailed description thereof, the foregoingdescription is intended to illustrate and not limit the scope of theinvention, which is defined by the scope of the appended claims.

Other aspects, advantages, and modifications are within the scope of thefollowing claims.

What is claimed is:
 1. A method for depositing a film of a material ontothe surface of a patterned substrate, the method comprising: i)dissolving a precursor of the material into a solvent to form asupercritical or near-supercritical solution; ii) exposing the patternedsubstrate to the solution under conditions at which the precursor isstable in the solution; and iii) mixing a reaction reagent into thesolution under conditions that initiate a chemical reaction involvingthe precursor, wherein the material is deposited onto the surface of thepatterned substrate when the substrate and the reaction reagent are incontact with the solution, while maintaining supercritical ornear-supercritical conditions.
 2. A method of claim 1, wherein thetemperature of the substrate is maintained at no more than 300° C.
 3. Amethod of claim 1, wherein the solvent has a reduced temperature between0.8 and 2.0.
 4. A method of claim 1, wherein the solvent has a densityof at least 0.1 g/cm³.
 5. A method of claim 1, wherein the solvent has adensity of at least one third of its critical density.
 6. A method ofclaim 1, wherein the solvent has a critical temperature of less than150° C.
 7. A method of claim 1, wherein the temperature of the substratemeasured in Kelvin is less than twice the critical temperature of thesolvent measured in Kelvin.
 8. A method of claim 1, wherein thetemperature of the substrate measured in Kelvin divided by the averagetemperature of the supercritical solution measured in Kelvin is between0.8 and 1.7.
 9. A method of claim 1, wherein the chemical reaction is areduction reaction.
 10. A method of claim 9, wherein the reactionreagent is hydrogen.
 11. A method of claim 1, wherein the chemicalreaction is an oxidation or hydrolysis reaction.
 12. A method of claim1, wherein the material comprises a metal.
 13. A method of claim 1,wherein the material comprises a semiconductor.
 14. A method of claim 1,wherein the material comprises an insulator.
 15. A method of claim 1,wherein the material comprises a mixture of metals.
 16. A method ofclaim 1, wherein the material comprises a metal oxide or a metalsulfide.
 17. A method of claim 1, wherein the substrate comprisessilicon or a fluoropolymer.
 18. A method of claim 1, wherein the solventcomprises carbon dioxide.
 19. A method of claim 1, wherein the averagetemperature of the supercritical solution is different from thetemperature of the substrate.
 20. A composition comprising a film of amaterial deposited on a patterned substrate by a method of claim
 1. 21.The method of claim 1, wherein the patterned substrate has submicronfeatures.
 22. The method of claim 21, wherein the features have anaspect ratio greater than
 2. 23. The method of claim 21, wherein thefeatures have an aspect ratio greater than
 10. 24. The method of claim21, wherein the material is deposited to conformally cover the features.25. The method of claim 1, wherein the substrate is a silicon wafer andthe material is palladium or a palladium alloy.